Torsion oscillators are widely used in laser printers, copiers, fax machines, bar code scanners, laser scanning projectors, laser radars and laser scanning sensors etc. The reflective mirror of the torsion oscillator swings back and forth periodically and deflects the incident light beam to form a scanning span or a scanning angle. The torsion oscillator can be driven by various principles, such as electromagnetic force, electrostatic force, or piezoelectric force. The span or angle of the oscillation can be controlled by adjusting the drive energy input to the torsion oscillator. In case of electromagnetic force, the torsion oscillator can be actuated by the Lorentz's force which can be generated by applying an alternating current to the conductive coils arranged around the movable mirror of the oscillator and perpendicular to the preset magnetic field. The torsion oscillator will oscillate at the same alternating frequency of the current. The scanning span is proportional to the energy level of the drive current or the intensity of the magnetic field and the oscillation frequency is typically the same as of the drive signal frequency.
The scanning span of the torsion oscillator should be kept to a constant to stabilize the projected image for applications such as laser printing. U.S. Pat. No. 6,838,661, “Torsion oscillator stabilization including maintaining the amplitude of the oscillator without changing its drive frequency”, discloses a control method based on determining resonance frequency during start-up of the torsion oscillator. Since the resonance frequency of the torsion oscillator depends on environmental factors such as ambient temperature, humidity and atmospheric pressure, the control method needs to determine the resonance frequency with iterative procedures involving sensing and computation during every start-up of the oscillator. In addition, the resonance frequency may also depend on the drive energy level; therefore, the procedures of determining resonance frequency and maintaining oscillation amplitude by altering drive energy level are coupled and complex processes which are time consuming and expensive.
In the application of a laser beam printer, the modulation of laser beam and the rotation of photosensitive optical pickup (OPC) drum need to be synchronized to ensure proper printing of image and text. If the scanning frequency of the torsion oscillator is adjusted due to change in operating condition such as temperature change, the rotation speed of the OPC drum may need to be adjusted accordingly for synchronization. Therefore, the drive motor speeds of the OPC drum and of the paper feeding mechanism, and the timing of laser modulation need to be controlled precisely that leads to increase of system complexity and cost.
FIG. 1 illustrates a conventional arrangement of a torsion oscillator 102 to scan an incident light beam (not shown), and a left sensor 100a, and a right sensor 100b to detect the scanned light beam for synchronization and/or scan amplitude control purposes. Light beams 106a and 106b are the incident light beam deflected by the torsion oscillator 102 at the left and the right extremes of the scanning angle θ0, respectively. The torsion oscillator 102 deflects the light beam in a sinusoidal motion along a projected scan trajectory shown as a dashed line in FIG. 1. Sensors 100a and 100b are positioned within the extremes of the projected scan trajectory and the effective or usable scan angle for image forming is limited within the span of the sensors.
FIG. 2 illustrates the waveforms of the sensing signals from both sensors 100a and 100b and the scanning angle of the deflected light beam. With the horizontal axis representing time and the vertical axis representing signal amplitude, diagram (A) and (C) shows the sensing signals of the sensors 100a and 100b, respectively. In diagram (B), the horizontal axis represents time and the vertical axis represents the amplitude of the scanning angle. Sensing signals 200 and 202 are generated when the deflected light beam passes through the left sensor 100a while sensing signals 204 and 206 are generated when the light beam passes through sensor 100b. The period of an oscillation cycle T is the sum of time intervals t1 and t2 defined by the intervals between the sensing signals. The effective scan span or usable scan angle θp is limited by the locations of the sensors 100a and 100b for imaging forming and is less than the full scanning span or the scanning angle θ0. For applications requiring constant scan angles, one need to determine the scan angle θ0 using both the time interval t1 or t2 and the effective scan span θp. A smaller effective scan span may lead to larger system dimension in order to obtain a projected scan trajectory with enough width.